A typical circuit board can include a variety of circuit board modules, i.e., components, such as integrated circuits (ICs), capacitors, resistors, connectors, and so on. These circuit board modules typically connect with a multi-layered board formed of conductive and non-conductive circuit board material (e.g., copper and fiberglass, respectively).
Some circuit boards include complex modules which have multiple components mounted on a miniature circuit board, i.e., a small section of circuit board material. Such a module is often called a multi-chip module (MCM) because it typically includes multiple ICs, i.e., multiple xe2x80x9cchipsxe2x80x9d. Some MCMs include components mounted exclusively on a top surface of the miniature circuit board (hereinafter called an xe2x80x9cMCM boardxe2x80x9d to distinguish it from the main circuit board onto which the MCM mounts). Other MCMs include components mounted on both a top surface and a bottom surface of the MCM board. Typically, contact members (e.g., pins, pads, etc.) of the MCM components connect with corresponding contact members (e.g., vias) of the MCM board at solder joints.
In a similar manner, modules (MCMs and non-MCMs) typically connect with circuit boards at solder joints. Some modules mount to circuit boards using ball grid array (BGA) technology. Mounting a module to a circuit board using BGA technology involves using a grid of solder balls (i.e., beads of solder) between pads of the module and corresponding pads of the circuit board. Applied heat melts the solder balls to form solder joints between the pads of the module and the circuit board.
In general, large circuit board fabrication facilities manufacture circuit boards on a large scale and under tightly controlled environmental conditions (i.e., temperature, humidity, etc.) in a highly automated manner (e.g., using large scale computer controlled automated equipment). Accordingly, manufacturing yields at such facilities are generally consistent and high.
Nevertheless, on occasion, a manufactured circuit board may operate improperly. In some cases, such improper operation may be due to a faulty module, i.e., a defective or improperly mounted module. A computerized circuit board analyzer may be able to test and identify the module causing the failure. In such a situation, a technician may be able to xe2x80x9creworkxe2x80x9d the circuit board by removing the faulty module and replacing it with a new one. To this end, the technician removes the faulty module using an assembly rework station. A typical assembly rework station includes a heated gas source, a vacuum source and a special nozzle that is adapted to fit over the faulty module which is mounted to the circuit board. In general, the technician lowers the nozzle over the module, applies heated gas through the nozzle to melt solder connections holding the module to the circuit board, and applies a vacuum (typically through a pipe in the middle of the nozzle) to lift the module from the circuit board once the solder connections have melted.
After the technician removes the failed module from the circuit board using the assembly rework station, the technician typically cleans out the installation location of the circuit board (e.g., removes any remaining solder debris), loads the nozzle with a new module (e.g., fastens the new module within the nozzle using the vacuum), and positions the nozzle holding the new module on the cleaned installation location. The technician then applies heated gas through the nozzle to thoroughly heat the module such that solder on contact members of the module (and perhaps additional solder placed at the installation location) melts to form new solder connections with the circuit board.
The nozzles of some assembly rework stations are configured to apply heated gas to a module, and apply cooler gas (e.g., room temperature gas) exclusively around a periphery of the module. In particular, as a technician operates the nozzle of such an assembly rework station to remove a faulty module from a circuit board or to install a new module onto a circuit board, the nozzle of the assembly rework station applies the heated gas to all parts of the module to melt solder between the module and the circuit board, and the cooler gas around the outside edges of the module to prevent the solder connections of the neighboring circuit board modules from re-melting or re-flowing.
Unfortunately, conventional assembly rework stations, which apply heated gas to install new modules on circuit boards, do not adequately protect the new modules against heat-related damage. For example, when a technician installs a new multi-chip module (MCM), i.e., a module formed by multiple components soldered to a miniature circuit board (an MCM board), onto a main circuit board, the application of heated gas to the new MCM can melt solder connections between MCM components and the MCM board. In some cases, the reflowing of solder can form unreliable cold solder joints between the MCM components and the MCM board. In more extreme cases, the reflowing of solder can result in components falling off the MCM board. Even if the module being installed is not an MCM, the module can sustain damage to internal circuitry due to the extreme temperatures of the heated gas. Such damage can be particularly costly when the new modules have already undergone thorough manufacturing and testing procedures prior to their installation on circuit boards during reworking of the circuit boards.
In contrast, the present invention is directed to techniques for installing a module on a circuit board by simultaneously heating a perimeter portion of the module, and bringing an inner portion of the module to a temperature that is lower than that of the perimeter portion. Heating the perimeter portion of the module melts solder disposed between contact members of the module and corresponding contact members of the circuit board in order to form solder connections. Bringing the inner portion of the module to a temperature that is lower than that of the perimeter portion reduces the likelihood of causing heat-related damage to the module itself.
One arrangement of the invention is directed to a module installation system for installing a module on a circuit board. The module has a perimeter portion and an inner portion. The module installation system includes a heating source, a cooling source, and a nozzle coupled to the heating and cooling sources. The nozzle is configured to simultaneously heat the perimeter portion of the module, and cool the inner portion of the module in order to install the module on the circuit board. Accordingly, contact members around the perimeter portion of the module can form solder connections with the circuit board while cooling of the inner portion of the module protects the inner portion from heat-related damage. For example, if the module is an MCM, solder joints connecting MCM components to the MCM board at the inner portion will be less likely to reflow and cause a failure.
In a preferred arrangement, the heating source provides a first fluid (a gas or a liquid), and the cooling source provides a second fluid. As such, the nozzle applies the first fluid to an area adjacent the perimeter portion of the module, and the second fluid to an area adjacent the inner portion of the module. In one arrangement, the first and second fluids are gases, e.g., nitrogen, which the nozzle preferably applies at the substantially the same pressure (i.e., +/xe2x88x9210%), e.g., each at four pounds of pressure per square inch (psi). In another arrangement, the first fluid is a gas and the second fluid is a liquid (e.g., a gel).
In one arrangement, the nozzle includes a housing that (i) contacts the inner portion of the module and (ii) defines a chamber through which the second fluid is capable of passing. In this arrangement, the housing operates as a thermal mass (or thermal capacitor) to keep the inner portion of the module cooler as the perimeter portion of the module is heated.
In one arrangement, the housing further defines multiple baffles that extend into the chamber. Additionally, the nozzle further includes an input port that defines an opening leading into the chamber, and an output port that defines an opening leading from the chamber. In this arrangement, the second fluid is capable of passing through the input port leading into the chamber, over the multiple baffles, and through the output port leading from the chamber. The baffles facilitate temperature transfer between the second fluid and the housing which contacts the inner portion. As a result, the inner portion of the module stays at a lower temperature than the perimeter portion of the module.
In another arrangement, the housing further defines a bleed orifice. Additionally, the nozzle further includes an input port that defines an opening leading into the chamber, and an output port that defines an opening leading from the chamber. In this arrangement, the second fluid is capable of passing through the input port leading into the chamber, over the bleed orifice, and through the output port leading from the chamber. As the second fluid passes over the bleed orifice, the bleed orifice directs some of the second fluid (e.g., a cool gas) over a particular area of the inner portion to provide an enhanced localized cooling effect to that area. This feature of the invention is particularly useful when installing MCMs. That is, the directed second fluid can provide enhanced cooling to a particular MCM component mounted on a surface of the MCM board in order to provide better cooling of that component.
In one arrangement, solder resides between contact members of the perimeter portion of the module and the circuit board. Preferably, the heating source provides the first fluid at a temperature that is higher than a melting point of the solder, the cooling source provides the second fluid at a temperature that is lower than the melting point of the solder.
Another arrangement of the invention is directed to a module installation system for installing a module on a circuit board using multiple heating sources. The module has a first portion and a second portion. The module installation system includes a first heating source, a second heating source, and a nozzle coupled to the first and second heating sources. The nozzle is configured to simultaneously (i) heat the first portion of the module to a first temperature, and (ii) heat the second portion of the module to a second temperature that is lower than the first temperature, in order to install the module on the circuit board.
In one arrangement, the first portion of the module is a perimeter portion of the module, and the second portion of the module is an inner portion of the module. In this arrangement, the nozzle preferably is configured to apply a fluid at the first temperature to an area adjacent the perimeter portion of the module, and a fluid at the second temperature to an area adjacent the inner portion of the module. This arrangement enables the nozzle of the module installation system to duplicate a heating profile which is typical of large furnace fabrication facilities that produce circuit boards on a large scale. The heating profile can include a heat soke phase in which both the inner and perimeter portions of the module are heated to stable temperatures with a stable temperature difference, i.e., the inner portion being heated to a stable temperature that is lower than that of the perimeter portion. The heating profile can further include a subsequent temperature spike that raises the temperatures of the perimeter portion (and perhaps the inner portion, but preferably to a lesser degree) in order to melt solder between the module and the circuit board. A benefit of this arrangement is that the temperature differential between the inner portion and the perimeter portion remains relatively close thus minimizing stresses and possible damage that could otherwise result from greater temperature differences.
The features of the invention, as described above, may be employed in electronic systems and related components such as those manufactured by EMC Corporation of Hopkinton, Mass.